It seems to depend on the time of day and the temp. Most of the time they sleep in their hut, but in the afternoon they are almost always on the top ledge sleeping where their top food bowl is (waiting for dinner as soon as they wake). They are normally also on a ledge more if it's warmer in the room. They do occasionally sleep in the hammocks but not as often as they use to.

All my boys are different. Wooden nest boxes, tubes, shelves and hanging houses are all favourites. The three boys who live together will start in the fleece hanger, and then all migrate to a shelf and then to the wooden nest box. Zulu likes to sleep on his fleece cover with his head resting against his pellet bowl, or on top of his nest box. Salt often sleeps on top of his fleece hanger.

Tom will sleep most places - from what I can tell it reflects on how safe he feels.

Hidey hut/house only when he's scared - he hasn't slept in it since the first week I had him.
Ledges - majority of the time he sleeps on these.
Fleecy hammock - occassionally sleeps here, when he's feeling lazy he'll sleep here cos he can reach his food bowl without getting out of the hammock
Anywhere on the floor - when he's really chilled out he's quite happy to stop and nap in the middle of the room, but he does sprint off if there's a sudden noise or movement.